The present invention relates to improvements in fixed-bed pressure gasification of coal or other solid carbonaceous fuel to produce fuel gas.
Processes for fixed-bed pressure gasification of coal are well known in the art. In one type of such processes, a pressurized gaseous mixture (commonly referred to as "blast gas" comprising water vapor and free oxygen (O.sub.2) is introduced into a lower zone (the ash zone) of a fixed bed of coal and passed upwardly through the bed, which is contained in a suitable gasifier. Coal is introduced into an upper portion of the gasifier and at least partially devolatilized in a devolatilization zone to liberate water, oil and tar from the introduced coal. The at least partially devolatilized coal is thereafter passed to and gasified in a reduction zone disposed below the devolatilization zone. Thereafter, the gasified coal is passed to an oxidation zone disposed below the reduction zone. Heat for the gasification is generated in the oxidation zone by combustion of the carbon remaining in the coal after the gasification thereof.
Thereafter, the ash byproduct produced in the oxidation zone is passed through an ash zone, wherein a stirrer-equipped grate reduces the size of large clinkers as may be required for discharging the coal ash from a lower portion of the gasifier.
The raw gaseous fuel stream exiting the devolatilization zone contains a mixture comprising water, tar and oil.
In practice of the above-described process, prior to end use of the raw gaseous fuel stream, it is subjected to clean-up operations wherein an aqueous liquor comprising water, tar and oil is formed. A gas clean-up system and method suitable for such operations are described by D. E. Woodmansee in U.S. Pat. No. 4,150,953, which is incorporated herein by reference. As described therein, an "oily aqueous blowdown liquid" results from quenching and washing in the clean-up of raw gaseous fuel exiting the gasifier. The integrated coal gasification power plant described therein includes not only the gas clean-up system, but also a water treatment plant for treatment of the waste water exiting the plant.
In the practice of the above-described process, stirring or agitation of the bed at one or more locations is advantageously employed. Thus, a vertically movable upper stirrer is desirably employed for agitating upper portions of the bed (e.g. in the oxidation and reduction zones) to break up cakes formed by gasification of highly caking coals. A second vertically movable stirrer which is rotatably operable in relatively lower and relatively upper positions is advantageously employed to both promote the discharge of the dry ash from the ash zone and agitate or stir the bed immediately under the combustion zone in the aforesaid relatively lower and upper positions, respectively. A grate-and-rabble arm construction wherein the rabble arm serves as the aforesaid second movable stirrer is described in U.S. Pat. No. 4,098,586 (D. E. Woodmansee), which is incorporated herein by reference.
It would be advantageous to have improvements in the above-described process and corresponding apparatus, wherein at least a substantial fraction of at least one of the volatile tar and oil contained in the raw gaseous fuel exiting the gasifier would be recovered therefrom, recycled to the gasifier without premature cracking, and cracked in the gasifier to form coke and fixed, normally non-condensable fuel gases (i.e., carbon monoxide and low molecular weight hydrocarbons having, e.g. from 1 to about 4 carbon atoms per molecule) with revaporization of at most a minimal portion of the tar or oil per se.
It has now been found by practice of the present invention that such improvements are provided, whereby the BTU-value of the resulting fuel gas can be increased in simple, efficient manner.